Fade resistant digital transmission and reception system

ABSTRACT

In a digital broadcast communications system, a higher priority component and a lower priority component are broadcast from a transmitter to a receiver. Each of these components generates a main and a supplemental signal, and each supplemental signal is advanced in time with respect to the corresponding main signal. The main and supplemental signals for both the higher and lower priority components are combined into a single signal, which is broadcast to a receiver. In the receiver, the time advanced supplemental signals are stored in a buffer to time align them with their corresponding main signals. Both main signals are processed in the normal manner in the receiver, and are also monitored to detect a fading event. When a fading event is detected, the corresponding buffered supplemental signals are substituted for the faded main signals and normal processing continues.

[0001] This application claims the benefit of U.S. ProvisionalApplication #60/(PU 010154) filed Jul. 19, 2001

BACKGROUND

[0002] 1. Field of the Invention

[0003] The present invention relates to an improved transmission andreception system for digital television. More particularly the presentinvention is provided to overcome any fading of signal that may occurbetween the transmission and reception of a digital terrestrial TVsystem.

[0004] 2. Discussion of the Related Art

[0005] Any terrestrial TV system must overcome a number of problems intransmitting signals to a receiver. For example, the United States hasadopted the Advanced Television System Committee (ATSC) system usingeight level vestigial side band (8-VSB) as its digital televisionstandard. Because the VSB system is a single carrier modulation system,it is susceptible to fading caused by multipath transmission and signalattenuation. These effects are well understood and the probabilitycharacteristics have been documented. If the fade is deep, wide and longenough in duration, the demodulation system in the TV receiver will losesynchronization and the signal will be lost. Such fading is particularlysevere in mobile reception of the signal used in digital television.

[0006] Attempts have been made to correct signal fading that isfrequency selective by using, for example, equalization techniques.However such techniques can result in degraded performance when fadingoccurs. Other techniques are not frequency selective.

[0007] One such solution to fading that has been presented is a“staggered multicasting” system which redundantly sends data in thedigital communication system to avoid the fading characteristics in aparticular channel. This system is described in provisional application,serial No. 60/(PU 010153) filed Jul. 19, 2001 by the same inventors asthe present application. The contents of this provisional applicationare incorporated herein by reference. This application disclosesrepeating the data stream at a period approximately equal to or greaterthan the statistically expected fade period value. A problem remains,however, in how to organize the redundant data in such a system foroptimum use.

[0008] Techniques are known that can vary the quality of service (QoS)and scalability characteristics of transmitted data. Such techniques arecommon in internet protocol streaming services, and rely on creatingpriorities in the network switches. QoS and scalability techniques maybe very useful in switched network broadcast systems. Clearly, however,no such switch network is provided in the television broadcast medium.Lost data packets in the television broadcast system are not caused bytraffic congestion, as in the internet, but rather by the lossy natureof the wireless channel.

[0009] The above mentioned provisional application disclosesbroadcasting redundant data in order to provide a level of guaranteedservice. The level of redundancy provided in the bitstream directlyaffects the error robustness of the system.

[0010] In an audio/video broadcast system, the audio channel is normallyprotected more robustly than the video channel. That is, the viewer canaccept a degraded video signal or even no video signal for a short timeperiod. However losing the audio is more disturbing to the listener.Therefore, a higher QoS level should be placed on the audio channel.Other arrangements of QoS levels may be desired.

[0011] The present invention seeks to produce such a beneficial systemby utilizing techniques that add additional robustness to a signalcomponent or channel which has a higher perceived importance to the user(e.g. audio vis-a-vis video). For example, the maximum fade durationwhich is overcome by the redundant data stream can be longer for higherpriority data than for lower priority data. The audio fade duration, forexample, can be supported for a larger time period than that supportedfor the video. In this case, this will cause the delay buffer to belarger for the audio channel but since the data rate of audio isrelatively small compared to video, it can be buffered for a low cost.

[0012] While the detailed description of the current invention belowfocuses on the details of the 8-VSB system, it must be recognized thatthe solution of the current invention is equally applicable to anydigital broadcast transmission system that is subject to a fadingchannel environment.

SUMMARY OF THE INVENTION

[0013] In accordance with principles of the present invention, in adigital broadcast communications system, a higher priority component anda lower priority component are broadcast from a transmitter to areceiver. Each of these components generates a main and a supplementalsignal, and each supplemental signal is advanced in time with respect tothe corresponding main signal. The main and supplemental signals forboth the higher and lower priority components are combined into a singlesignal, which is broadcast to a receiver. In the receiver, the timeadvanced supplemental signals are stored in a buffer to time align themwith their corresponding main signals. Both main signals are processedin the normal manner in the receiver, and are also monitored to detect afading event. When a fading event is detected, the correspondingbuffered supplemental signals are substituted for the faded main signalsand normal processing continues.

BRIEF DESCRIPTION OF THE DRAWING

[0014]FIG. 1 is a block diagram of a transmitter incorporating theprinciples of the present invention.;

[0015]FIG. 2 is a block diagram of a receiver incorporating theprinciples of the present invention;

[0016]FIG. 3 is an illustration of groups of audio and video packetswith different fade redundancies;

[0017]FIG. 4 is an illustration of groups of audio and video packetswith different fade redundancies as well as an extra audio redundancy;and

[0018]FIG. 5 is an illustration of groups of audio and video packetsillustrating audio plus scalable video with fade redundancies.

DETAILED DESCRIPTION

[0019]FIG. 1 is a block diagram of a transmitter incorporating theprinciples of the present invention. In the illustrated embodiment, thetransmitter operates in accordance with the provisions of the AdvancedTelevision Standards Committee (ATSC) digital television standard datedSep. 16, 1995 which is incorporated herein by reference. However, oneskilled in the art will understand that the principles of the presentinvention are applicable to any communications system in which thechannel is subject to fading.

[0020] Video source material is applied via a terminal 10 to MPEGencoders 20 and 30. These encoders provide video signal encoding andcompression in accordance with MPEG standards. The output from theencoder 20 is applied via conductor 21 to one input of a transportmultiplexer 40. The encoder 30 processes the data stream in the samemanner as encoder 20 but its output is applied via a conductor 31 to apacket buffer delay 32. The output of the delay 32 is applied to anotherinput of the transport multiplexer 40. The video signal is encoded intorespective digital data streams. The encoding can utilize known bit ratereduction methods and compression techniques which are appropriate forthe particular signals involved. The compressed video data streamsprovided from the encoders 20 and 30 may also be divided into packetscontaining the encoded video information as well as data identifyingeach packet.

[0021] Audio signals are applied via a terminal 11 to a digital audiocompressor (DAC)12. The digital audio compressor 12 processes the audiosignals into digital signals as will be subsequently illustrated and theoutput thereof is applied to a further input of the transportmultiplexer 40. From the terminal 11 the audio signals are also appliedto a second digital audio compressor 13. The compressed data signalsexiting the compressor 13 are applied to a delay circuit 14 and fromthere to a fourth input of the transport multiplexer 40.

[0022] The respective encoded video and audio signals are thenmultiplexed into a single data stream by the transport multiplexer 40.Additional data signals could also be supplied to the multiplexer 40 toprovide e.g. control data subsequently utilized in the digital TVreceiver.

[0023] The output from the transport multiplexer 40, containing the foursets of video and audio signals, is channel coded and modulated by thechannel coding sections 50, the symbol mapping section 60 and the mixer70 utilizing the carrier insertion circuit 80. These circuits alsoinsert the various “helper” signals that will aid the 8-VSB receiver inaccurately locating and demodulating the transmitted RF signals. Theseinclude the ATSC pilot, segment sync and frame sync signals.

[0024] The output signals from the mixer 70, modulated in the 8-VSBmanner, are broadcast to receivers and appear in the form shown in FIG.3. As has been indicated above, the audio signals are considered thehigher priority signals while the video signals are considered the lowerpriority signals. In the upper portion of FIG. 3 the two audio or higherpriority signals 301 and 302 are shown. As will be discussed below, theupper bitstream 301, encoded by encoder 12, is the supplemental streamwhich is sent in advance timewise with respect to the main audio signal302, encoded by encoder 13 and delayed by delay circuit 14. In the lowerportion of FIG. 3, two video or lower priority signals 303 and 304 areshown. As with the audio signals 301 and 302, the video signal 303,encoded by encoder 20, is considered the supplemental video signal whilethe video signal 304, encoded by encoder 30 and delayed in the delaycircuit 32, is considered the main video signal.

[0025] The respective main and supplemental low priority video signalsand the main and supplemental high priority audio signals aresubstantially identical to each other except that the main signals aredelayed in time with respect to the supplemental signals. It is clearthat this is accomplished for the video signals in the buffer 32 whilethe audio signals are delayed by the delay circuit 14 of FIG. 1.

[0026] Referring now to FIG. 2 a schematic diagram for a VSB receiverincorporating the principles of the present invention is illustrated. Inthe 8-VSB transmitted signal, the eight levels of the transmitted signalare recovered by sampling only the I-channel or in-phase information, ina known manner. In FIG. 2 the received signal is demodulated byreversing the processes that were applied in the transmitter. That is,the incoming VSB signals are received, downconverted, filtered and thendetected. The segment sync and the frame sync are then recovered. Thisis accomplished by the mixer 100, the local oscillator 101, the low passfilter 102, the analog-to-digital converter 103, the mixer 104 and thecarrier recovery circuit 106 as well as the interpolator 107 and thesymbol timing recovery circuit 108.

[0027] The output of the interpolator 107 is applied to the equalizer110. The segment sync signal aids in the receiver clock recovery whilethe field sync signal is used to train the adaptive ghost-cancelingequalizer 110. One of the advantages of the VSB system is that complexequalization is not necessary since the equalizer operates only on theI-channel or real information.

[0028] The output of the equalizer 110 is applied to a forward errorcorrection circuit (FEC)120. This circuit provides forward errorcorrection signals which are applied to and utilized in a transportdemultiplexer 130. The FEC circuit 120 also provides a signal toindicate that it was unable to properly decode its input signal. Theoutputs from the transport demultiplexer 130 mirror the inputs to thetransport multiplexer 40 in the transmitter illustrated in FIG. 1. Thesesignals include the supplemental video signal on conductor 131, the mainvideo signal on conductor 132, the main audio signal on conductor 133and the supplemental audio signal on conductor 134.

[0029] The supplemental video or low priority signal is applied to abuffer delay 150 having a delay which equals the delay of the buffer 32in the transmitter while the main video signal is applied on conductor132 directly to a stream select circuit 140. Similarly the main highpriority or audio signal is applied on conductor 133 directly to thestream select circuit 140 while the supplemental audio signal is appliedto a delay circuit 136 having a delay equal to the delay of the delaycircuit 14 in the transmitter. The delayed supplemental video signal isapplied from the buffer 150 to the stream select circuit 140 while thedelayed supplemental audio signal is applied from the delayed circuit136 to the stream select circuit 140. Consequently, the main andsupplemental signals for both the audio high priority and video lowpriority signals are applied to the stream select circuits aligned intime.

[0030] The stream select circuit 140 normally selects as outputs one ofthe respective main and supplemental audio and video signals to besupplied to the decoder 160 for application to the display processingcircuits and display device 180.

[0031] If a fading event occurs, the buffered supplemental signals willbe selected by the stream select circuit 140. Such a fading event isdetermined by the error detector circuit 121 connected to respectiveoutputs of the forward error correction circuit 120 and the transportdemultiplexer 130. The occurrence of a fading event in either the mainhigh priority signal or the main low priority signal may be detected bya number of different possible measures in the physical layer. Morespecifically, a measure of the signal quality of the received signal maybe monitored to detect a fading event. For example a signal-to-noiseratio detector may be used which will detect a decrease in thesignal-to-noise ratio should the amplitude of the processed main signalsdecrease. Alternatively, the bit error rate of the received signal maybe monitored to detect if it drops below a predetermined level, or thepacket error signal from the FEC 120 may be monitored to detect anyundecodable packets. One or more of these indications may be monitoredby the error detection circuit 121 to detect a fading event. When thecircuit 121 determines that the main signal is corrupt it instructs thestream select circuit 140 to utilize the supplemental channel data.

[0032] The supplemental data will continue to be used until either therespective buffer is exhausted or the receiver recovers and the mainchannel is restored to above its threshold. It is evident that once theVSB receiver recovers it must stay recovered long enough to permit thesupplemental buffer to refill to be prepared for another fade event inthe respective main stream signal. The size of the buffered delays of150 and 136 can be based on the expected fade duration of the respectivehigh and low priority signals. For example such delay can be between 5ms and a few seconds.

[0033] Referring once again to FIG. 3, these illustrations indicatedifferent video and audio fade duration redundancies in the design. FIG.3 is a time diagram illustrating the timing of packets transmittedthrough the communications channel. In the supplemental audio signal301, a first packet is labeled ‘a’, a second packet is labeled ‘b’, andso forth. During the time period 310, the audio packet buffer 150 isloaded with the initial supplemental audio packets. It can be seen thatthe supplemental audio signal 301 has been advanced in time about tendata packets when compared to the main audio signal 302, thus, duringtime period 310 audio packets ‘a’-‘j’ are loaded into buffer 150. In asimilar manner, during time period 312 supplemental video signal 303packets are loaded into buffer 136. However the supplemental videosignal 303 has been advanced in time only about four data packets withrespect to the main video signal 304.

[0034] At time t1, the first main audio packet ‘A’, corresponding tosupplemental audio packet ‘a’, is received. Audio packet ‘A’ is followedby the next main audio packet ‘B’, corresponding to supplemental audiopacket ‘b’, and so forth. Similarly, at time t2, the first main videopacket ‘A’, corresponding to supplemental video packet ‘a’, is received,followed by the next main video packet is ‘B’, corresponding tosupplemental video packet ‘b’, and so forth. In the normal mode ofoperation, the main audio and main video packets are selected by thesignal selector 140 and processed by the subsequent receiver circuitry.

[0035] Time period 314 represents a fading event lasting for a threepacket time intervals. During time interval 314, main audio packets ‘H,‘I’ and ‘J’, main video packets ‘H’, ‘I’ and ‘J’, supplemental audiopackets ‘r’, ‘s’ and ‘t’, and supplemental video packets ‘l’, ‘m’ and‘n’ are all lost. Time period 316 represents a time interval where thesignal is back to full strength and the receiver is reacquiring thesignal, i.e. the demodulator chain is resynchronizing and the forwarderror correction circuitry is recovering. During time interval 316, mainaudio packets ‘K’, ‘L’ and ‘M’, main video packets ‘K’, ‘L’ and ‘M’,supplemental audio packets ‘u’, ‘v’ and ‘w’, and supplemental videopackets ‘o’, ‘p’ and ‘q’ are all lost.

[0036] Because the audio buffer 150 contains 10 supplemental audiopackets, the supplemental audio packets ‘h’-‘m’, transmitted during timeinterval 318 in advance of the corresponding main audio packets ‘H’-‘M’and before the fading event 314-316, are in the audio packet buffer 150at the time of the fading event 314-316. Thus, the six main audiopackets ‘H’-‘M’ lost in the fading event can be recovered by using thesupplemental audio packets ‘h’-‘m’ from the audio packet buffer 150.However, because the video buffer 136 contains only four supplementalvideo packets, transmitted in advance of the main video packets, thevideo channel is only partially protected. That is, the fade duration ofsix packets is greater than the four packet advance of the videosupplemental signal. Therefore, the video data packets ‘L’ and ‘M’ ofsignal 304 will be lost and no corresponding supplemental packets areavailable to replace them. As has been noted above, the delay buffer inthe audio channel is larger than the delay buffer in the video channel.However since the data rate of audio is relatively small compared tovideo, the extra buffering of the audio signal has a relatively lowcost.

[0037] Shadings are provided in FIG. 3 to aid in understanding thedrawings. Thus the shading 306 indicates the packets decoded at thereceiver, shading 307 indicates packets lost due to the fading event.The shading 308 indicates packets lost due to the receiverre-acquisition and the lack of shading shown in 309 are indicative ofpackets which are received but not used.

[0038] It should be clear that, after a fading event, the overall systemis vulnerable to fades until the supplemental buffers that have beenused have been repleted. This is because all the streams can be lost inthe fade. Additional advanced supplemental streams might be used to rideout multiple close successive fades. This however will consume morebandwidth.

[0039] Referring now to FIG. 4, an example is shown wherein the audiochannel has the same maximum fade duration as that in FIG. 3. The videodata streams 403 and 404 are substantially the same as that shown inFIG. 3. However the audio supplemental channel 401 has two copies ofeach packet in the main audio channel 402. That is, for each main audiopacket, e.g. ‘A’, two corresponding supplemental packets, e.g. ‘a’, onereceived at time t3 and a second one at time t4, are received and storedin the audio buffer 150. There are still ten supplemental audio packetsstored in the buffer 150, received during time period 406.

[0040] In the example of FIG. 4, two fading events happen relativelyclose to each other. The first includes a fade at time period 408 andreceiver recovery period at time period 410, and the second includes afade at time period 412 and receiver recovery period at time period 414.It can be seen that in the main video signal 404 some packets, e.g.‘h’-‘k’ are made available from the secondary video signal 403. Howeverboth the fade duration and the two fades in a row have created lostpackets, e.g. ‘L’, ‘M’, ‘O’, ‘P’, ‘Q’, ‘S’, ‘T’, in the video. In theaudio channel however the two copies of audio data packets arranged inthe supplemental channel 401 enable the receiver to recover all of themissing data due to both fades. It should be noted that in thearrangement shown in FIG. 3 if two fades such as those of FIG. 4occurred, the audio channel would not have survived without loss. It isclear, therefore, that having multiple copies of the audio channelpackets in the supplemental signal is highly advantageous. Again asnoted above this is available at relatively low additional cost becausethe data rate of audio is relatively small compared to that of video.

[0041] It should be noted that in the arrangement of FIG. 4, the maximumdistance between any two redundant packets still defines the longestfade time. However with additional redundant packets the multiple fadeevents can also be concealed in the high priority signal stream.

[0042] The same principles discussed above can be utilized to helpprotect the video channel. Scalable encoding in the video channel canprovide a graceful degradation characteristic. The specific type ofscalable encoding is not essential. It could be spatial, temporal, SNR,or fine grain scalability. Scalable video coding involves creating twoseparate video bitstreams: a base layer including data needed to form animage with the lowest acceptable quality; and an enhancement layerincluding data which, when combined with the base layer data, creates ahigher quality image. If the base layer is protected with redundancy forfades while the enhancement layer has no such redundancy then a gracefuldegradation is provided from a higher quality image to a lower qualityimage when a fading event occurs. One skilled in the art will understandthat more than two layers of video may be generated and encoded fordiffering fade event durations according to principles of the presentinvention.

[0043] In FIG. 1, the main video signal generated by the encoder 30includes both the base layer information over line 31 and enhancementlayer information, illustrated by the dotted line 35, while thesupplemental video signal generated by encoder 20 includes only baselayer information over line 21. Similarly, in FIG. 2, the enhancementlayer information from the main video signal, shown as dotted line 135,is applied to the stream selector 140 in the same manner as the baselayer information from the main video signal on conductor 132, whileonly base layer information for the supplemental video signal issupplied to the delay circuit 150. Therefore, the enhancement layerinformation is in time synchronism with the main base layer information.Consequently, under normal conditions, a higher quality image may beproduced from the base layer and enhancement layer information of themain video signal.

[0044]FIG. 5 illustrates signals produced by such a system. The audiochannels 501 and 502 are substantially the same as 401 and 402 of FIG.4. However, the video channels 503 and 504 relate only to the base layerinformation. That is, in FIG. 5, the base layer main video signal 504has fade duration redundancy of four packet duration due to the additionof a corresponding time advanced base layer supplemental video signalpacket stream 503. However, the enhancement video layer 506 does nothave a corresponding supplemental packet stream, and therefore has nofade redundancy. In the arrangement shown in FIG. 5 the fading event attime interval 510 and the recovery period at time interval 512 causesthe loss of packets ‘L’ and ‘M’ from the base layer video data and allof packets ‘H’-‘M’ from the enhancement layer 506. The audio signal doesnot lose any packets. Consequently, the high resolution video will bedegraded down to base layer resolution during most of the fading event,but a picture is still provided during that portion and the audio isstill decoded properly. As noted above it is loss of audio that will bemost noticed by a viewer of television. The viewer can accept somedegradation in the video without causing any problems.

[0045] It is clear from the above examples that many differentarrangements are possible. The tradeoff must be made between theduration of the fade and the size of the buffer used. Also the bit ratemust be traded off with the redundancy. Clearly if more redundancy isused, then fewer bits are available for the application. The method andapparatus described above provides different fade duration redundanciesfor different bitstreams to create levels of QoS on a wireless lossychannel. That is, higher priority audio data is given higher level offading event resistance than the lower priority video data. Additionalredundancy may be provided to further protect high priority data, e.g.audio data, from successive fades. The application of staggeredmulticasting to scalable video bitstreams create graceful degradationduring fading events as illustrated in the bitstream illustration inFIG. 5. While the present invention has been described with respect toparticular embodiments and particular illustrative examples, it isevident that the principles of the present invention may be embodied inother arrangements without departing from the scope of the presentinvention as defined by the following claims.

1. A method for improving the reception of a transmitted signalcomprising the steps of: producing a main set and a supplemental set ofhigh priority data from a first source in a transmitter; delaying saidmain set of high priority data in time with respect to said supplementalset of high priority data; producing a main set and a supplemental setof low priority data from a second source; delaying said main set of lowpriority data in time with respect to said supplemental set of lowpriority data; transmitting a signal carrying said main and delayedsupplemental sets of both said high priority and low priority data forreception by a receiver; applying said main set of high priority datareceived in said receiver to normal high priority data receptionchannels of said receiver; storing said supplemental set of highpriority data received in said receiver in a buffer for high prioritydata in said receiver; applying said main set of low priority datareceived in said receiver to normal low priority data reception channelsin said receiver; storing said supplemental set of low priority datareceived in said receiver in a buffer for low priority data in saidreceiver; detecting an undesired change in said transmitted signal;substituting corresponding portions of said supplemental high prioritysignal stored in said buffer for any undesirably changed portions ofsaid main high priority signal; and substituting corresponding portionsof said supplemental low priority signal stored in said buffer for anyundesirably changed portions of said main low priority signal.
 2. Amethod as claimed in claim 1 wherein said transmitted signal istransmitted in the form of a VSB signal.
 3. A method as claimed in claim1 wherein said main set of high priority data is delayed for a longertime period than said main set of low priority data.
 4. A method asclaimed in claim 1 wherein said high priority data is audio signal dataand said low priority data is video signal data.
 5. A method as claimedin claim 4 wherein said supplemental set of audio signal data containsmultiple copies of said main audio signal data.
 6. A method as claimedin claim 5 wherein said supplemental set of audio signal data containstwo copies of said main audio signal data.
 7. A method as claimed inclaim 4 wherein the low priority data is composite video signal datahaving both a base layer and an enhancement layer and wherein saidsupplemental set of video signal data contain only said base layer andthe main set of video data contains both the base layer and theenhancement layer.
 8. A method as claimed in claim 7 further comprisingthe step of combining said received enhancement layer of said videosignal data with said received base layer of said video signal data torestore the composite video signal data in said receiver.
 9. A method asclaimed in claim 1 wherein said undesired change in said receivedsignals is related to a quality of said received signal and said changeis detected by monitoring a quality measure of said received signal. 10.A method as claimed in claim 9 wherein said quality measure is one ormore of a signal-to-noise ratio, bit error rate or packet error ratemeasure.
 11. A method for improving the reception of a signal carrying afirst set of synchronously encoded first priority signals and a secondset of synchronously encoded second priority signals, said secondpriority being lower than said first priority, each of said first andsecond sets containing a main signal and a supplemental signal, saidmain and supplemental signals being staggered in time with each saidsupplemental signal being in advance of the corresponding saidrespective main signal, said supplemental signal of said first prioritysignal being advanced by a larger time interval than said supplementalsignal of said second priority signal, comprising the steps of: storingeach of said supplemental signals in respective buffers in saidreceiver; processing each of said main signals in said receiver in anormal manner; detecting an undesired change in the received signal; andsubstituting corresponding portions of said stored supplemental signalsfor any undesirably changed portions of said main signals.
 12. A methodas claimed in claim 11 wherein the received signal is a VSB modulatedsignal.
 13. A system for improving the reception of transmitted signalscomprising: means for producing a main and a supplemental set of highpriority data from a first source in a transmitter; first delay meansfor delaying said main set of high priority data in time with respect tosaid supplemental set of high priority data; means for producing a mainand a supplemental set of low priority data from a second source; seconddelay means for delaying said main set of low priority data in time withrespect to said supplemental set of low priority data; means fortransmitting a signal carrying said main set and said delayedsupplemental set of both the high and low priority data; a receiverhaving respective normal receiving channels for low and high prioritydata; means for applying said main set of high priority data received insaid receiver to said normal high priority receiving channel of saidreceiver; a first buffer circuit for high priority data in saidreceiver; means for storing said supplemental set of high priority datareceived in said receiver in said first buffer circuit; means forapplying said main set of low priority data received in said receiver tosaid normal low priority receiving channel in said receiver; a secondbuffer circuit for low priority data in said receiver; means for storingsaid supplemental set of low priority data received in said receiver insaid second buffer circuit; detector circuits in said receiver fordetecting any undesired changes in said received signal; means in saidreceiver for substituting corresponding portions of said supplementalset of high priority data stored in said first buffer circuit for anyundesirably changed portions of said main set of high priority data; andmeans in said receiver for substituting corresponding portions of saidsupplemental set of low priority data stored in said second buffercircuit for any undesirably changed portions of said main set of lowpriority data.
 14. A system as claimed in claim 13 wherein thetransmitting means comprises circuitry for transmitting a VSB signalcarrying said main set and said delayed set of both the high and lowpriority data.
 15. A system as claimed in claim 13 wherein said firstdelay means provides a longer time delay than said second delay means.16. A system as claimed in claim 13 wherein said high priority data isaudio signal data and said low priority data is video signal data.
 17. Asystem as claimed in claim 16 wherein said supplemental audio signaldata contains multiple copies of said main audio signal data.
 18. Asystem as claimed in claim 17 wherein said supplemental audio signaldata contains two copies of said main audio signal data.
 19. A system asclaimed in claim 16 wherein said video signal data produced in saidtransmitter is a composite video signal data having both a base layerand an enhancement layer and wherein said main and supplemental sets ofvideo signal data both contain said base layer of said composite videosignal data and the main set of video signal data further contains theenhancement layer.
 20. A system as claimed in claim 19 wherein combiningmeans in said receiver combines said enhancement layer of said videosignal data with said base layer of said first video signal to restoresaid composite video signal data in said receiver.
 21. A receiver forimproving the reception of signals transmitted in the form of a firstset of synchronously encoded first priority signals and a second set ofsynchronously encoded second priority signals, said second prioritybeing lower than said first priority, each of said first and second setscontaining a main signal and a supplemental signal, said main andsupplemental signals being staggered in time with said supplementalsignal being in advance of said respective main signal, said receivercomprising: a first buffer circuit for storing said supplemental signalof said first priority signals received in said receiver; a secondbuffer circuit for storing said supplemental signal of said secondpriority signals received in said receiver; signal processors forprocessing each of said main signals received in said receiver in anormal manner; a detector circuit for detecting any undesired change issaid received signal; and means coupled to said detector circuit andsaid first and second buffer circuits for substituting correspondingportions of said stored supplemental signals for any undesirably changedportion of said respective main signals.
 22. A receiver as claimed inclaim 21 wherein said received signal is a VSB modulated signal.
 23. Areceiver as claimed in claim 21 wherein said undesired change in saidreceived signal is related to a quality of said received signal and saiddetector circuit monitors a quality measure of said received signal. 24.A receiver as claimed in claim 23 wherein said quality measure is one ormore of a signal-to-noise ratio, bit error rate or packet error ratemeasure and the detector monitors one or more of a signal-to-noiseratio, bit error rate or packet error rate measure.